Mobile Dual-Comb Detects Methane Leaks

Illustration showing how trace gases are detected in the field using a mobile dual-frequency comb laser spectro­meter. The spectro­meter sits in the center of a circle which is ringed with retroreflecting mirrors. Laser light from the spectro­meter passes through a gas cloud, strikes the retro­reflector and is returned directly to its point of origin. (Source: S. Sizemore & I. Coddington, NIST)

Accurately detecting, locating and quantifying leaks of methane, the main component of natural gas and a major fuel source worldwide, is criti­cally important for both environ­mental and economic reasons. Unfor­tunately, tradi­tional methods are slow, labor-inten­sive, limited to small coverage areas and expensive to operate over time. Now, researchers at the National Institute of Standards and Tech­nology NIST, the University of Colorado Boulder and the National Oceanic and Atmo­spheric Admini­stration NOAA have success­fully demonstrated a novel solution that features an cost-effec­tively monitor leaks of methane and other trace gases with extreme precision and over large areas.

Although its lifetime in the atmo­sphere is much shorter than the more prevalent greenhouse gas, carbon dioxide, methane is about 25 times more powerful than CO2 because it traps more of the sun’s heat pound for pound. Taking that dif­ference into account, it is estimated that the methane leaked or inten­tionally vented from human acti­vities such as natural gas explo­ration, storage and pipe­lining, coal mining, chemical manu­facturing and processing, waste manage­ment, and livestock farming accounts for about 10 percent of the warming potential of all green­house gas emissions in the United States.

Current approaches to methane detection rely heavily on inspec­tors using infrared cameras to look laboriously for gas plumes one by one across large sites, such as well fields with hundreds of potential leaks. This is time consuming, requires skilled operators and may only be done once a year or less because of the high cost for moni­toring expansive, remote or otherwise difficult-to-survey areas. Aircraft and vehicle-mounted IR cameras or spectro­meters offer another option; however, this method also is expensive and may be ill-suited for continuous moni­toring.

In fact, said NIST physicist Ian Coddington, potential users of current methane detection and quanti­fication systems often find the costs prohi­bitive for meeting three needs: coverage of large areas, frequent moni­toring and sensi­tivity. “The last one really becomes evident when you realize that leaks may have rates as small as 0.2 cubic meters per hour, about one-fourth the respira­tion rate of an average human being,” he said. To address these problems, NIST and its partners have developed a novel observing system that combines the world’s first “in-the-field” dual-frequency comb laser spectro­meter and an array of corner cube retro­reflectors.

The spectro­meter is based on the frequency comb tech­nology. The new trace gas monitoring system uses an evo­lution of the frequency comb tech­nology, a dual-comb laser spectrometer where a second comb is added. “When paired, the combs can act like hundreds of thousands of laser spectro­meters working in unison, and yield a device that is 10 to 100 times better than a traditional spectro­meter and very sensitive to leaks, even at a great distance,” Coddington said. In the demon­stration, the researchers ringed a Colorado well field containing multiple methane leak sources with retro­reflectors equally spaced along a 1-kilo­meter-diameter circle. In the center of the ring, they stationed a trailer with the portable dual-comb device inside.

To detect and quantify methane leaks within a “pie-slice” section of the circle pulses from the dual-comb laser are first bounced off one reflector back to the spectro­meter to quantify residual gas from outside the field. Sub­tracting that amount from the measure­ment made by the second reflector yields the level of methane being released within the arc alone. Data from repeated scans of the sector under varying wind condi­tions are inter­preted by a computer model to define the leak locations and rates.

According to Kuldeep Prasad, a NIST mechanical engineer, combining local meteoro­logical data with the measure­ments taken from all of the arcs within the ring of retroreflectors provides a complete accounting of the methane escaping from an entire field of wells. “The monitoring can be done conti­nuously over time so that every leak in a field is accurately detected and quantified,” he said. “Further­more, if you set up mobile spectro­meters at adjoining fields, you can drama­tically expand the total coverage area to several square kilo­meters.”

For gas storage faci­lities, oil and gas processing plants and other indus­trial environ­ments where line-of-site leak moni­toring isn’t possible, Prasad said that the spectro­meter can be placed high above the site to detect and quantify methane as it rises. Not only does the inno­vation allow for 24/7, highly reliable methane leak detection and quanti­fication over large areas in the field, but it makes the entire operation more cost-effective than current IR methods.

Besides refining the moni­toring capabi­lities of the mobile dual-comb laser spectro­meter, our team will next be attemp­ting to bring the operating cost down to a per-well-per-year price tag of around $300”,  Prasad said. “Achieving that goal would make the system affor­dable for most users.” Cod­dington said that the portable dual-comb laser spectro­meter system for detecting and quanti­fying methane leaks already has attracted several industrial partners to evaluate the system’s perfor­mance at opera­tional oil and gas faci­lities. (Source: NIST)

Reference: S. Coburn et al.: Regional trace-gas source attribution using a field-deployed dual frequency comb spectrometer, Optica 5, 320 (2018); DOI: 10.1364/OPTICA.5.000320

Link: Precision Laser Diagnostics Laboratory, University of Colorado Boulder, Boulder, USA

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